1. Field of the Invention
The present invention relates to the conversion of solid biomass to liquid fuels and specialty chemicals.
It has long been recognized that biomass, in particular biomass of plant origin, is abundantly available and is a potential source of liquid fuels and valuable chemicals. See, for example, “Energy production from biomass”, by P. Mc.Kendry—Bioresource Technology 83 (2002) p 37-46, and “Coordinated development of leading biomass pretreatment technologies” by C. E. Wyman et al, Bioresource Technology 96 (2005) 1959-1966.
Refined biomass materials, such as vegetable oils, starches, and sugars, can almost completely be converted to liquid fuels such as biodiesel (methyl or ethyl esters of fatty acids) and ethanol. However, the use of these refined biomass materials as starting points for liquid fuels diverts precious food resources from animal and even human consumption. This makes these starting materials expensive, and also meets with ethical objections.
It is far more desirable to find a way for converting non-edible biomass to liquid fuels and valuable chemicals, in particular if this non-edible biomass does not at present have an economic use and is therefore considered “waste”. Examples of such biomass materials include agricultural wastes, such as bagasse, straw, corn stover corn husks and the like. Other examples include forestry wastes, such as wood chips and saw dust from logging operations or waste from paper and/or paper mills. What these materials have in common is that they contain significant amounts of lignocellulose and crystalline cellulose, making them resistant to chemical conversion and to fermentation. It is known that the biomass consists of three main components being lignin, amorphous hemi-cellulose and crystalline cellulose, assembled in such a compact manner that makes it less accessible and therefore less susceptible to chemical and/or enzymatic conversion.
2. Description of the Related Art
Various processes have been proposed for converting non-edible biomass to liquid fuels, animal feeds, and chemicals. Generally speaking, these processes fall into one of the following categories:
Hydrothermal Upgrading (Htu); See References                “Process for the production of liquid fuels from biomass” by Van den Beld et al WO 02/20699 A1        “Developments in direct thermochemical liquefaction of biomass 1983-1990” by D. C. Elliott et al., Energy & Fuels 1991, 5, 399-410        “A literature survey of intermediate products formed during the thermal aqueous degradation of cellulose” Polym. Plast. Technology. Eng 11, (2), 127-157 (1978)        
Pyrolysis; See Reference                “Pyrolysis of Wood/Biomass for Bio-Oil: A critical Review” by D. Mohan et al., Energy & Fuels 2006, 20, p 848-889        
Gasification (Followed by Fischer Tropsch Synthesis).                “Chemical Processing in High-pressure Aqueous Environments—Development of Catalysts for Gassification” by D. C. Elliott et al, Ind. Eng. Chem. Res. 1993, 32, 1542-1548        
Acid Hydrolysis                Schmidt et al “Hydrolysis of biomass material” US2002/0117167A1        
Enzymatic Fermentation.
See for instance reference: “A review of the production of ethanol from softwood” by M. Galbe et al., Biomedical and Life Sciences, vol 59, no6, September 2002
Hydrothermal Upgrading (HTU) refers to processes whereby biomass is reacted with liquid water at elevated temperature (well above 200° C.) and pressure (50 bar or higher). The high temperatures and pressures that are needed to obtain suitable conversion rates make these processes expensive, requiring special high pressure equipment constructed with special metal alloys which for commercial plants, are difficult to operate and have relatively short life times. In addition, the products obtained in HTU processes are heavily degraded because of polymerization and coke formation that take place under the prevailing reaction conditions. The liquid products obtained by HTU processes tend to be highly acidic and corrosive, and unstable.
Pyrolysis generally refers to processes carried out at high temperatures (500 to 800° C.) in the absence of oxygen, or with so little oxygen present that little or no oxidation takes place. The resulting liquid products are of poor quality, heavily degraded, and low pH, and require extensive (hydro-) treatment for upgrading to transportation fuels or chemical feedstocks.
It is desirable to develop a process for converting biomass under much milder conditions than prevail in the traditional HTU and pyrolysis processes, in part to avoid the high cost of equipment necessary for operating under these conditions, and in part also to avoid the product degradation taking place under these more severe reaction conditions.
Gasification of biomass, followed by FT synthesis, is inherently expensive as it involves a complete breakdown of hydrocarbon material, followed by a synthesis of different hydrocarbons. This route involves a complex multi-step and therefore costly processing scheme.
Any economic biomass conversion process must be aimed at preserving the chemical structures present in the biomass as much as possible, to the extent consistent with the goal of making liquid fuels. The overall scheme should be simple and low in capital as well as operating costs.
Enzymatic fermentation is capable of converting only a relatively small portion of the available cellulose in biomass, generally on the order of 40%. The process is slow, requiring 24 hours or more per batch and operates best at low solid to liquid ratios. Accordingly, the process must be carried out in large fermentation vessels. The enzymes used in these processes are expensive when compared to the cost of chemicals used in chemical conversion processes.
Acid hydrolysis has been proposed as a precursor to enzymatic fermentation. The purpose is to provide an initial breakdown of (ligno-)cellulose, so that more of it is available for subsequent fermentation. Acid hydrolysis is carried out under atmospheric conditions and at temperatures below 100° C. The handling of large quantities of acid makes this process unattractive, in particular because the acid must be either removed or neutralized before the fermentation step. The formed salts adversely affect the subsequent fermentation process.
There is a need for a low-cost process that is able to convert a large proportion of the (ligno-)cellulosic material present in non-edible biomass under conditions that are mild enough to avoid high equipment and energy costs and/or substantial degradation of the conversion products.
Biomass in general represents a composite comprising mainly lignin, amorphous hemi-cellulose and crystalline cellulose, which are assembled in a strong compact form, which due to the lack of accessibility is resistant to chemical treatments, impregnation, and dissolution. It is highly desirable to develop means to dissociate these three main components from the composite and develop susceptibility which will allow chemical reactions and subsequent conversions to take place. Our invention provides means to accomplish this by using a two step process: a) Breaking down/dissociation of the components within the composite and developing susceptibility. b) Reacting the individual components appropriately using physical, mechanical, thermal and chemical means for the efficient conversion to fuels and chemicals.